43
JOHNSON CONTROLS
FORM 145.05-NOM2 (807)
Coil Leaving Temperature Control (FlexSys Units)
On FlexSys units the control will compare the supply
air leaving the evaporator coil and compare it to 40.0°
F; and generate an analog output based on the difference
and the amount of response to a given change in output.
This is a reverse acting loop. As the temperature drops
below 40.0° F, the damper output increases.
FLEXSYS BYPASS DAMPER CONTROL
If the unit is programmed to control the Slab Temperature
in either the cooling or economizer mode the unit will
also employ a return air bypass damper. The purpose of
the return air damper is to bypass return air around the
cooling coil in order to temper the supply air leaving the
coil to prevent the supply air entering the conditioned
space from being too cool. The control tries to maintain
the supply air temperature no lower then 2.0° F below
the
“AC / ECON / S/A SP”
set point.
The control also measures the pressure drop across the
evaporator coil. The control will also modulate the by
pass damper to prevent the pressure drop across the
evaporator coil from dropping below the
“MIN DX
PRS DROP”
The following parameters must be programmed through
the
“Setpoints”
section of the Keypad / Display or by
a communicated input.
“MIN DX PRS DROP”
– The control will modulate
the by pass damper to keep the pressure drop across the
evaporator coil above this set point. The default value
is 0.5 inches WC.
“AC-S/A DEW EN”
– This input instructs the control to
use the slab dew point temperature in determining the
DX cooling set point. The default value is ON
Sequence of Operation
The control modulates the by pass damper to keep the
supply air temperature entering the condition space
above the
“AC / ECON / S/A SP”
set point minus 2.0°
F. As the supply air temperature drops below this value
the control will open the damper to by pass additional
return air around the evaporator coil, resulting in an
increase in the supply air temperature. The maximum
allowable damper by pass is 70%.
The control also monitors the pressure drop across
the evaporator coil. This is done to verify that there
is always an adequate amount of return air across the
evaporator coil. As the pressure drop across the coil
decreases below the
“MIN DX PRS DROP”
set point
the control will over ride and close the by pass damper
to maintain adequate airfl ow across the evaporator coil.
This could result in a temporary decrease in the mixed
supply air temperature.
HEATING OPERATION
The following parameters must be programmed through
the
“Setpoints”
section of the Keypad / Display or by
a communicated input.
“HEAT- S/A TEMP EN”
– The unit can control the
operation of the heating function by either the supply
air temperature or the return air temperature. If supply
air control is desired, this value must be turned ON. ON
is the default value.
“HEAT- S/A TMP SP”
– This is the set point the
controller will control to if the unit is confi gured for
“HEAT-S/A TEMP EN”
. The default value is 110.0°
F.
“HEAT- R/A TMP EN”
- The unit can control the
operation of the heating function by either the supply
air temperature or the return air temperature. If return
air control is desired, this value must be turned ON. ON
is the default value.
It is strongly recommended that both
supply air and return air be used for
all heating applications.
In order to prevent simultaneous
operation of the heating and cooling
functions, the “COOL-R/A TMP SP”
should be set at least 3.0 °F above the
“HEAT-R/A TMP SP”.
“HEAT- R/A TMP SP”
– This is the set point the
controller will control to if the unit is confi gured for
“HEAT- R/A TEMP EN”
. The default value is 72.0°
F.
5
Summary of Contents for YSCD
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